Can you sniff the traffic to find what's actually being sent? Is it sending any auth data at all and it's incorrect or being presented in a form the server doesn't like, or is it never being sent by firefox at all?

I'd recommend checking out the iMacros addon for Firefox. I use it to login to a local web server and after logging in, navigate directly to a certain page. The code I have looks like this, but it allows you to record your own macros:

@travis Looks very nice! I will sure take a look into it. I can think of several places I can use that

I never got round to sniff the traffic but found out that a php site on my own server with http-auth worked fine, so i figured it was something with delicious. I then created a php page that does a wget of the delicious api and everything works fine :)